At present, flat panel or planar types of display panel utilizing liquid crystal are in widespread use. Of the various types of liquid crystal display panel, the "active matrix" type of matrix display panel has been found to offer significant advantages for application to large-area displays having a high display element density. With an active matrix display panel, an individual switching element is provided to control each of the liquid crystal display elements. These switching elements can comprise three-terminal devices, e.g., thin-film transistors, or two-terminal elements such as diodes or non-linear resistance elements. Two-terminal switching elements have the advantages of ease of manufacture of the display panel, together with stability of the switching element characteristics. However although considerable advances have been achieved in producing highly compact display panels having low power consumption, through the use of liquid crystal, very little progress has been achieved in producing input devices such as keyboards which will offer similar advantages when utilized in compact, portable electronic equipment.
Various methods of performing input of data through a display device have been proposed and implemented in the prior art. The principal method has been the use of a light pen, containing a photoelectric transducer element, which is touched to the surface of a cathode ray tube to be activated by light emitted from the CRT. The position on the CRT face which is touched by the light pen can be determined from the timings at which various points on the display are activated to emit light. Another method which has been proposed for data input utilizing a display is to utilize strain gauges to detect the positions of points on the display to which pressure is applied by touching the display face.
However such prior art methods are not suitable for utilizing a liquid crystal matrix display panel as a highly compact device for both display and input of data. That is to say, since no light is emitted by a liquid crystal display panel, a conventional type of light pen cannot be utilized. Furthermore the strain gauge method referred to above can provide only a low level of resolution, and requires the use of an additional unit which must be manufactured separately from the body of the display panel. In addition, it is desirable that the resolution for input of data utilizing a display panel, i.e. the density of data input detection points, should increase in accordance with any increase in the density of display elements in the panel.